This invention relates to a composite electroplating film to be utilized as a wear-resistant surface layer of a sliding member, e.g., the plain bearing, which is used in automobiles, ships, aircraft, electric machinery, OA equipment, machine tools, industrial machinery in general, etc.
Well-known examples of such a sliding member include a multi-layer product which is composed of: a steel backing-metal layer, a bearing copper alloy or aluminum alloy layer, an intermediate layer, and a lead alloy surface layer. U.S. Pat. Nos. 2,605,149 and 3,180,008, and JP-Bl-39-22498, etc. disclose alloys to be used as the material for the lead alloy surface layer of such a multi-layer product. In these alloys, the amount of the alloying elements (which are to be added to the lead), such as tin, copper, indium, etc., is changed in various ways with a view to improving their wear resistance and fatigue resistance characteristics. Such alloys are actually being used at the present time.
The improvement attained by these conventional alloys, however, is not sufficient for sliding members used in present day automobile engines. This is because, due to the demand for cost reduction, it has become a common practice today to use in an automobile engine a shaft which is made of cast iron, such as spheroidal graphite cast iron (e.g., JIS FCD70). The problem with such a cast iron shaft is that the surface layer of a receiving bearing of a multi-layer wears at an early stage because of projections (burrs) or the like existing around graphite areas on the shaft surface. As a result, part of the intermediate layer or of the bearing alloy layer is exposed, which rapidly causes seizure phenomenon. Even if the surface layer does not wear so badly as to cause seizure phenomenon, changes are caused in the gap (oil clearance) between the shaft and the bearing, which can cause noise and vibrations. Apart from this, the general tendency today is that a steel shaft for general use is used under high speed and high load conditions, so that the surface layer of a receiving bearing of a multi-layer is also subject to abrasion at an early stage, thus giving rise to a problem which is similar to that mentioned above.